FLOOD06021

• County: Larimer
• Stream Name: Big Thompson
• Basin: South Platte
• Title: Flood History of the Big Thompson River Basin: Flash Floods, Paleofloods, and Dam-Break Floods
• Date: 1/1/1979
• Prepared For: Larimer
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

Table 1. Comparison of Manning's n values
Main Channel
Location Year Peak Slope Hydraulic Manning n Manning n Percent
of discharge (ft/h) radius (field (equation 1) differences
flood (ft3/,) (ft) selected) in n values
Big Thompson River
tributary below
Loveland Heights.
Colo. . . . . . . . . .. .. . . .. . . . .. .. . . 1976 8.700 0.066 7.0 0.040 0.102 -155
Big Thompson River
above Drake.
Colo.. . . ... . . . . . . .. . . . .. . . . ... 1976 28.000 .022 8.5 .045 .065 -44
Canacadea Creek at
Alfred Station,
N.Y. ......................... 1972 6.080 .012 5.0 ,040 .057' -42
Chenunda Creek at
Stannards. N.Y. ................ 1972 9.200 .008 6.7 .037 .046 -24
Cataract Creek near
Basin. Mont. . . . . . ... . .. . . . . . .. . 1981 3.160 .046 5,0 .070 .094 -34
Cleghorn Canyon at
Rapid City.
S. Oak. . . . . . . . . . . . . . . . . . . . .. . . 1972 12.600 .019 7.0 .041 .063 -54
Average -59

V
F =.ygD; F = 1.0

where V = the mean velocity;
g = acceleration due to gravity; and
D = mean depth of flow.

If high-water marks above a steep drop or fall (prefer-
ably bedrock) are assumed to represent critical depth,
then the solution for peak discharge Q, based on one
cross section, is:

Q=A ygD

where A = cross-sectional area.

Because the flow is at or near critical throughout
the higher gradient channel, the exact location of the
critical depth section is not that important, although it
is best to locate the cross section immediately upstream
from the drop. Also, because the least scour generally is
at or just upstream from a bedrock drop, problems as-
sociated with scour are minimized. One can obtain im-
proved results from equation 3 by surveying a second
cross section located a minimum of 2 1/2 times the
mean depth upstream (Barnes and Davidian, 1978) and
by balancing the energy equation for the cross sections
for the observed water-surface elevations. Step-back-

~7

(2)

water analysis (Bailey and Ray, 1966) is then made to
develop a rating curve at the upstream (approach) cross
section. The discharge then can be determined from the
observed approach water-surface elevation and the
rating curve.
For example, the critical-depth method (equation
3) was applied to two higher gradient sites in the Big
Thompson River basin for which slope-area measure-
ments were made after the 1976 flood (McCain and

EXPLANATION
Yl DEPTH BEFORE EROSION
Y2 DEPTH AFTER EROSION

(3)

Bedrock

Figure 2. Hypothetical cross section showing how high-
water marks could be set before extensive erosion of valley
fill. This erosion would result in excessive slope-area dis.
charge measurements.

Evaluation of the Slope-Area Method for Computing Peak Discharge 17

l.J~~1.

w'S? '2. '3 10